chore(nyc coverage): fix coverage fail on async/await

- nyc fails to cover async await function, to correct this
- install babel-plugin-istanbul
- add it as a test environment requirement in .babelrc
-

.babelrc

@@ -1,4 +1,9 @@
 {
     "presets": ["@babel/preset-env"],
-    "plugins": ["@babel/transform-runtime"]
+    "plugins": ["@babel/transform-runtime"],
+    "env": {
+        "test": {
+          "plugins": [ "istanbul" ]
+        }
+    }
 }

package.json

@@ -33,6 +33,7 @@
         "@babel/runtime": "^7.4.4",
         "@sendgrid/mail": "^6.4.0",
         "aws-sdk": "^2.458.0",
+        "babel-plugin-istanbul": "^5.1.4",
         "bcrypt": "^3.0.6",
         "cookie-parser": "~1.4.3",
         "cors": "^2.8.5",
@@ -56,5 +57,9 @@
         "nyc": "^14.1.1",
         "should": "^13.2.3",
         "supertest": "^4.0.2"
+    },
+    "nyc": {
+        "sourceMap": false,
+        "instrument": false
     }
 }

yarn.lock

@@ -939,6 +939,15 @@ aws4@^1.8.0:
   resolved "https://registry.yarnpkg.com/aws4/-/aws4-1.8.0.tgz#f0e003d9ca9e7f59c7a508945d7b2ef9a04a542f"
   integrity sha512-ReZxvNHIOv88FlT7rxcXIIC0fPt4KZqZbOlivyWtXLt8ESx84zd3kMC6iK5jVeS2qt+g7ftS7ye4fi06X5rtRQ==
 
+babel-plugin-istanbul@^5.1.4:
+  version "5.1.4"
+  resolved "https://registry.yarnpkg.com/babel-plugin-istanbul/-/babel-plugin-istanbul-5.1.4.tgz#841d16b9a58eeb407a0ddce622ba02fe87a752ba"
+  integrity sha512-dySz4VJMH+dpndj0wjJ8JPs/7i1TdSPb1nRrn56/92pKOF9VKC1FMFJmMXjzlGGusnCAqujP6PBCiKq0sVA+YQ==
+  dependencies:
+    find-up "^3.0.0"
+    istanbul-lib-instrument "^3.3.0"
+    test-exclude "^5.2.3"
+
 balanced-match@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/balanced-match/-/balanced-match-1.0.0.tgz#89b4d199ab2bee49de164ea02b89ce462d71b767"